The present application relates to metal oxide nanoparticles, a production method thereof, and a light-emitting element assembly and an optical material that include the metal oxide nanoparticles.
In nanoparticle-resin composite materials prepared by adding nanoparticles of a metal oxide such as titanium oxide to a polymer, their refractive indices can be controlled to be higher than the refractive index nm of the polymer alone. Therefore, such metal oxide nanoparticles serving as an additive for a polymer are useful for designing and producing various optical materials. Examples of optical products in which an increase in refractive index is effective include optical lenses, light control films, Fresnel lenses, antireflective coatings, optical discs, diffusion films, and holographic substrates.
In the metal oxide nanoparticles used in such applications, it is important that the transparency and other optical properties of the nanoparticle-resin composite materials prepared by combining a polymer are not degraded. When the refractive index np of the metal oxide nanoparticles and the refractive index nm of the polymer are different from each other, the transparency of the nanoparticle-resin composite material in which the optical path length is, for example, on the millimeter order markedly depends on the particle diameter of the metal oxide nanoparticles and the dispersibility of the metal oxide nanoparticles in the polymer. When the metal oxide nanoparticles have a large particle diameter, the transparency of the nanoparticle-resin composite material is decreased because the metal oxide nanoparticles scatter light. When the dispersibility of the metal oxide nanoparticles is not satisfactory, the transparency of the nanoparticle-resin composite material is markedly decreased because the metal oxide nanoparticles are agglomerated, and the agglomerated metal oxide nanoparticles strongly scatter or reflect light.
A method of synthesizing titanium oxide nanoparticles, the surfaces of which are coated with a surfactant, is disclosed in, for example, P. D. Cozzoli, A. Kornowski, and H. Weller; J. Am. Chem. Soc. 2003, 125, 14539 (hereinafter referred to as Document 1). The surfactant on the surfaces of the nanoparticles stabilizes the dispersion of the titanium oxide nanoparticles to prevent agglomeration thereof. Therefore, it is believed that such a surfactant is also effective for stabilizing the dispersion of the titanium oxide nanoparticles in a polymer.
Typical crystal structures of titanium oxide include rutile and anatase. Rutile-type titanium oxide has a refractive index higher than that of anatase-type titanium oxide and has excellent light resistance. Accordingly, from the standpoint of high refractive index, titanium oxide nanoparticles used in a nanoparticle-resin composite material are preferably composed of rutile-type titanium oxide. The thermodynamically stabilized phase of bulk titanium oxide is rutile-type titanium oxide at room temperature and at atmospheric pressure. However, when the particle diameter is 10 nm or less, the specific surface area of the particles significantly increases, and anatase-type titanium oxide becomes the stabilized phase (see H. Zhang and J. F. Banfield; J. Mater. Chem. 1998, 8, 2073 (hereinafter referred to as Document 2)).
It is known that even when the particle diameter is 10 nm or less, rutile-type titanium oxide can be obtained by doping tin with titanium oxide. For example, according to Japanese Examined Patent Application Publication No. 4-27168, rutile-type TixSn1-xO2 nanoparticles having a particle diameter of 10 nm or less can be produced by a hydrothermal treatment of a mixed gel of a titanium compound and a tin compound. Furthermore, Japanese Patent No. 2783417 discloses a method of producing rutile-type TixSn1-xO2 nanoparticles having a particle diameter of 10 nm or less in which hydrous titanium oxide serving as an intermediate is heated in the presence of a tin compound. Japanese Unexamined Patent Application Publication No. 2005-132706 discloses a method of producing rutile-type TixSn1-xO2 nanoparticles including a reaction of a titanium compound and a tin compound at a pH in the range of −1 to 3.
In the method disclosed in Document 1, in order to produce titanium oxide nanoparticles having a particle diameter of 10 nm or less, it is necessary to set the concentration of the starting material to a low value, and it takes several days to conduct the reaction. Consequently, the production cost is increased. Furthermore, the method disclosed in Document 1 provides not rutile-type but only anatase-type titanium oxide nanoparticles. Document 2 does not disclose a liquid-phase synthesis of rutile-type titanium oxide nanoparticles which have a particle diameter of 10 nm or less, the surfaces of which are coated with a surfactant, and which are stably dispersed in an organic solvent.
In the production process disclosed in Japanese Examined Patent Application Publication No. 4-27168, an autoclave is necessary for the hydrothermal treatment, thereby increasing the production cost. In the technique disclosed in Japanese Patent No. 2783417, in order to achieve an excellent dispersion stability of particles, heating of the particles together with a silicon compound is performed. However, since the refractive index of the silicon compound is not high, the refractive index of TixSn1-xO2 nanoparticles is disadvantageously decreased as a whole. In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-132706, although the diameter of resulting primary particles is 10 nm or less, as a result of agglomeration of the primary particles, the entire particle diameter exceeds 10 nm.